1. Field of the Invention
The present invention relates generally to the sensing of currents and in particular to methods and apparatus for sensing current flowing through devices which exhibit resistive voltage-current characteristics when such devices function as switches.
2. Description of the Related Art
Control of functions within a circuit frequently depend upon the accurate sensing of the amount of current flowing through at least a portion of the circuit. For example, one such circuit is a switching mode power supply. Current-mode control of switching mode power supplies is a popular technique because such control provides speed and stability advantages. In order to provide current-mode control, however, it is necessary to sense the instantaneous current in the switching device of the switching mode power supply. Such sensing can be grouped into three categories, namely, resistive sensing, magnetic sensing and source-sensing.
With reference to FIG. 1, a simplified resistive sensing circuit 100 within a power supply circuit is illustrated. The sensing circuit 100 includes a sense resistor 102, a power switch 104 connected between sense resistor 102 and an impedance 106, an unregulated power input 108 connected to impedance 106, and a voltage sense output 114 which is coupled to a comparator (not shown). Impedance 106 could, for example, be a transformer, inductor or motor drive circuit. The comparator compares the potential of voltage sense output 114 to a known quantity and in response generates a signal to thereby regulate the amount of power provided to impedance 106. As will be appreciated by those skilled in the art, such a signal regulates, for example, the duty cycle (i.e., the percentage of on-time) and/or the frequency of the switching action of power switch 104. Other variables (such as the potential of the unregulated power and the value of impedance 106) being constant, an increase in the duty cycle increases the amount of power provided to impedance 106.
In further detail, sense resistor 102 is generally a low inductance and highly-linear resistor which is connected in series with impedance 106. Sense resistor 102 generates a voltage drop proportional to the current through power switch 104 where, according to Ohm's law, the potential of the voltage drop is equal to the product of the current through sense resistor 102 and the resistance of sense resistor 102. This simple arrangement is very attractive theoretically, but in actual use, the arrangement is plagued with disadvantages.
First, since the sense resistor 102 conducts the entire amount of current through power switch 104, the resistive sensing arrangement inherently results in a large power dissipation, or stated differently, the sense resistor 102 generates a large amount of heat from power which otherwise would be available to impedance 106. Although such power dissipation can be decreased by decreasing the resistance of sense resistor 102, such a change also attenuates the amplitude of the sense signal provided to voltage sense output 114. Because of the nature of the waveform generated by power switch 104 (basically a square wave, with residual spectral components due to reactive elements within switch 102 and impedance 108) the sense signal is also noisy, and thus an attenuation of its amplitude makes it more difficult for the comparator to operate properly. Thus, resistive sensing inherently requires a trade-off of sense signal amplitude against power dissipation. In addition, resistive sensing requires a precise, accurate, non-inductive resistor since the current through the sense resistor is calculated by dividing the sense voltage by the sense resistance. Furthermore, to the extent a non-inductive resistor has residual inductance, the sense voltage would also vary as a function of the spectral content the switched current. Although precision non-inductive resistors are commercially available, they are more expensive than ordinary wirewound resistors. Furthermore, the value of precision non-inductive resistors varies over temperature and time, thereby making the accuracy of current sensing by resistive sensing a function of temperature and age of the sense resistor.
Another method of sensing switching current is known as magnetic current sensing. With reference now to FIG. 2, a simplified magnetic current sensing circuit 200 is shown. The magnetic current sensing circuit 200 utilizes a current transformer 202 having a primary 204 connected in series with an impedance 206 and a power switch 208. One end of a secondary 210 of transformer 202 is, together with one end of power switch 208, connected to ground (also referred to herein as "common") while the other end of secondary 210 is provided to a voltage sense output 212. Both the primary 204 and the secondary 210 are wound about a core 214. Core 214 typically consists of a closed magnetic circuit formed of ferrite or laminated magnetic steel. Impedance 206 is also coupled to an unregulated power input 216, while voltage sense output 212 is coupled to a comparator (not shown). Such a comparator compares the potential of the voltage sense output 212 to a known quantity and in response generates a signal to thereby regulate the duty cycle of the current provided to impedance 206. A resistor 218 is coupled from the voltage sense output 212 to ground, and operates to insure that the primary 204 presents a low impedance. The resistor 218 also operates to convert current flowing through secondary 210 into a voltage. As with the resistive sensing arrangement previously described, such a signal regulates, for example, the duty cycle and/or frequency of power switch 208.
In operation, the secondary 210 develops a sense voltage which is proportional to the current flowing through primary 204. The magnetic sensing method is more accurate than the resistive sensing method because of a relative lack of thermal rise and because the conversion ratio for the sense signal can be expressed as a real number as opposed to a complex function, as in the case of a resistor which has nonlinearities. In addition, since transformer 202 can be of a step-up type (i.e., a greater number of windings in the secondary 210 than in the primary 204), the sense signal can be amplified. Thus, by utilizing a primary 204 with a low impedance (both resistive and reactive) a relatively large signal can be provided to sense voltage output 212 while the voltage drop across primary 204 is minimized.
Magnetic sensing, however, suffers disadvantages. A suitable transformer adds significant cost to a design. In addition, such transformers are difficult to emplace during automated assembly.
Recently, because manufacturers of DMOS (diffused metal-oxide semiconductor) power transistors offer packages containing a power switching device and a matched, smaller companion sense device, switching mode power supplies using current-mode control commonly rely upon "source sensing". Source sensing (also known as "sense FET" sensing) is a method which is based upon the fact that devices which are integrated into a common environment (such as a single chip of silicon) exhibit nearly identical electrical characteristics.
With reference now to FIG. 3, a simplified source sensing circuit 300 is shown. Source sensing circuit 300 includes a switching device 302 connected in series between an impedance 304 and ground. A sense device 306 has its drain connected to the drain of switching device 302. Sense device 306 also has its gate connected to the gate of switching device 302. The source of sense device 306 provides a sense current to a sense lead 308 which is coupled to a comparator (not shown). An unregulated power input 310 provides power to impedance 304.
In further detail, switching device 302 and sense device 306 are fabricated on a single chip to form a high current DMOS power transistor and a matching low-current sense device, respectively. Because devices 302 and 306 are fabricated as arrays of a fundamental cell, the ratio of drain-source currents through each device at any given gate-source voltage is equal to the ratio of the number of cells contained in the array for each device.
In a switching mode power supply which utilizes such DMOS devices, switching device 302 conducts the switch current, while sense device 306 conducts a fraction of the switch current in response to a gate drive voltage, V.sub.g, at a gate drive input 312. Since the gates of the sense device and the switching device are connected in common to the same potential (the gate drive voltage which is generated in part by the comparator), if the potential of the source of each device is also equal, then the ratio of the sense device current to the switching device current is known (based upon the respective physical dimensions of each device), and the current in the sense device can be taken as representative of (a known fraction of) the switch current. For this arrangement to be useful, however, it is desireable that the source potential of the sense device tracks with the source potential of the switching device. Such tracking can be implemented by one of two methods. Each of these methods can be distinguished by the connection made to the source of the sense device. One method can be described as "resistive multiplication" while the second method can be described as "virtual earth sensing".
Referring now to FIG. 4, shown is a simplified diagram for source sensing circuit 400 which employs resistive multiplication 400. In circuit 400 a switching device 402 has its gate connected in common with the gate of a sense device 404. The drain of switching device 402 is connected in common to the drain of sense device 404 and to an impedance 406. The impedance 406 is coupled to an unregulated power input 408. The source of switching device 402 is connected to ground while the source of sense device 404 is connected in common to one end of a sense resistor 410 and a first input 412 of a single supply amplifier 414. A second input 416 of operational amplifier 412 is connected in common with a second end of resistor 410 to ground. A voltage sense output 418 of amplifier 414 provides a sense voltage which is used by control circuitry (not shown) to drive a gate drive input 420 of devices 402 and 404.
In operation, since the source of switching device 402 is hard-grounded (i.e., connected directly to ground) and the source of sense device 404 is coupled to ground through sense resistor 410, sense resistor 410 generates a voltage (whose potential is directly proportional to the current through sense resistor 410) which can be detected amidst noise generated the circuit 400 and its associated circuitry, but which potential is not so large as to destroy the approximate parity between the gate-source voltages of sense device 404 and switching device 402. Thus, the source voltage of sense device 404 reflects a fraction of the switch current of switching device 402, multiplied by the value of resistor 410, hence the name "resistive multiplication". The voltage developed across sense resistor 410 is then amplified by amplifier 414 and is utilized by control circuitry to drive the gate of switching device 402 to thereby switch the current flowing through impedance 304.
Theoretically, the resistive multiplication of source sensing has the advantages of low power dissipation (because of the low amount of current flowing through sense resistor 410, in contrast to that of sense resistor 102 of FIG. 1) and of simplicity since the additional components over the circuit 300 are sense resistor 410 and single-supply amplifier 414. However, in practical application, such a circuit 400 does not perform sufficiently well for many applications. Basically, the theoretical simplicity afforded by circuit 400 cannot be realized with a two-component, single-ended sense circuit. The first reason is that with an actual circuit the level of noise generated by such a circuit interferes with the sense voltage. The second reason is that since current mode control must operate at high speed, a single-supply operational amplifier may not have a sufficient bandwidth to sufficiently amplify the sense voltage to permit control of the switching device. For if a control loop is to operate in the tens of kilohertz range, the bandwidth of the remaining circuitry must be in the megahertz range.
With reference now to FIG. 5, a virtual earth sensing circuit 500 is now described. Circuit 500 includes a switching device 502 and a sense device 504. A gate drive input 506 is connected in common to the gates of devices 502 and 504, while an impedance 508 is connected in common to the drains of devices 502 and 504. Impedance 508 is coupled to an unregulated power input 510. A non-inverting input 512 of an operational amplifier 514 is connected to a special voltage sensing pad 516. The source of switching device 502 is connected to ground. The source of sense device 504 is connected to an inverting input 518 of operational amplifier 514. A feedback resistor 520 couples an output 522 of operational amplifier 514 to inverting input 516.
In operation, the virtual earth source sensing circuit 500 provides greater accuracy and speed than the circuit 400 since the potential of the source of sense device 504 more closely tracks that of the source of switching device 502. This closer tracking is achieved by configuring operational amplifier 514 in a transresistance circuit. In further detail, the source of sense device 504 is connected to a summing node (inverting input 518) and a reference node (non-inverting input 512) is connected to the sensing pad 516. Feedback provided through resistor 520 maintains equality between the voltages at the summing and reference nodes. Operational amplifier 514 converts the current flowing through sense device 504 to a voltage at output 522. Thus, the virtual earth sensing circuit 500 provides faster and more accurate response than the resistive multiplication circuit 400. The disadvantage of virtual earth sensing circuit 500 is its sensitivity to noise, which results from the bandwidth required of operational amplifier 514 to respond to quickly changing voltages at its summing node (inverting input 518). In addition, the operational amplifier 514 requires a dual power supply in order to provide the required bandwidth.
Accordingly, it would be desireable to provide methods and apparatus for sensing currents which methods and apparatus provide improved noise immunity, low power dissipation and accuracy with a single supply operational amplifier.